This invention relates to a roll for use in casting metal products and an associated method, and more specifically to a roll defining an enclosed space which contains a working fluid which is converted from liquid to vapor phase and back again respectively to remove heat from the roll outer surface.
Roll casting, such as single roll casting, twin roll casting or melt spinning for example, is a well known method of producing metal products such as metal foil and strip. Roll casting is used to cast steel, aluminum, copper and other metals.
In most roll casting operations, molten metal is introduced onto the surface of the rotating roll. The rotating roll removes heat from the molten metal, causing the molten metal to solidify into a cast metal product such as foil or strip. There are numerous examples of roll casting machines in the prior art, such as those disclosed in U.S. Pat. Nos. 4,489,773; 4,502,528; 4,794,977 and 4,842,040.
In all of the above-cited patents, and in roll casting in general, heat is removed by providing a coolant that circulates in the hollow roll. For example, U.S. Pat. No. 4,794,977 teaches that the coolant is supplied to the core of the roll from an outside source and is guided to the outer surface by guide means. After performing its transfer function at the inner surface of the outer shell, the coolant is directed into the core and is exhausted therefrom.
There are several limitations inherent in these so-called "open systems". First, a strict design for sealing and mechanical couplings is required for safety and maintenance reasons. Second, the coolant, because it does not change phase from liquid to vapor, must be kept at a low temperature in order to perform its heat exchanging role. This, however, causes a large thermal gradient (metal to coolant) through the roll which induces thermal stresses that accelerate roll damage and shorten roll life. Third, because the heat extraction rate is limited, thinner roll walls are used which weaken the strength of the roll and which may result in roll deformation. Finally, it is difficult to maintain uniform circumferential temperature near the roll surface.
Thus, what is needed is a roll design that avoids the limitations of the prior art but which provides excellent heat extraction to produce quality cast metal products.